The present invention relates to the field of electrooptic signal processors.
Fiber optic delay lines are valuable for a variety of optical signal-processing applications. Prior work by Taylor et al (C. E. Lee, R. A. Atkins, and H. F. Taylor, "Reflectively Tapped Optical Fiber Transversal Filters," Electronic Letters, vol. 23, 596, 1987) has shown how fixed delays can be incorporated into a fiber. Additional work by Taylor et al has shown that Fabry Perot (FP)resonators made from glass fiber can be spliced into a fiber transmission line, and that the transfer-characteristic of those FPs can be altered by thermal means, thereby making the FP reflective or transmissive. See: S. Gweon, C. E. Lee, and H. F. Taylor, "Wideband Fiber Optic Signal Processor," IEEE Photonics Technology Letters, vol. 1, December 1989; H. F. Taylor, "Method for Producing Reflective Taps in Optical Fibers and Applications. Thereof," U.S. Pat. No. 4,848,999, issued July 1989. However, this work was limited because the thermal technique takes milliseconds to reconfigure the FP.
There is a need for fiber-compatible FP elements that can be reconfigured in microseconds or nanoseconds. FP elements are needed that have low optical loss and that couple efficiently in an end-fire manner to fibers or to on-chip waveguides. A variable-delay system comprised of optical fibers and fast FP elements is needed. A fiber system is required that contains a large number of electrically programmed elements that can be used to select one or several optical time-delay paths, and/or to insert signals onto the optical carrier. In this variable reflector system, we require reflection of an optical pulse at an addressed location on the guided optical path, while the unaddressed locations are transparent. Unbiased elements are needed that allow total transmission of the pulse to the next programmable element.
Some problems and limitations are also evident in previous approaches to optical time-delay steering of phased-array antennas: multiple glass fibers must be cut to exact lengths within less than 1 mm tolerance. Multiple optical sources or detectors are sometimes used to feed one antenna element, the extinction ratio of optical switches in this application tends to be less than 25 dB, the switches tend to be lossy, and individually fabricated control elements tend to be non-uniform.